<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>岁月</title>
    <script src="./base.js"></script>
    <script>
        window.onload = function () {
            var year = $('.year');
            year.onkeyup = function () {
                var regExp = /^\d{4}$/;//定义正则表达式

                if (year.value.match(regExp)) {//匹配
                    console.log('正确');
                } else {
                    console.log('错误');
                }
            }
        }
    </script>
</head>

<body>
    <input type="text" class="year">
</body>

</html>